We all get incredible technology, stop having children, all humans eventually dwindle down terribly, but by the time it is realized, it is too late. Our technology, without anyone to control it, destroys the world. That would be cool.

I think I'd like to see it end like it does in Nowhere Boy.
Where the Earth just sort of starts to fall apart and things break off and float away. Suddenly, but gently. And for a while, you can still live like you normally do, watching the planet die. Surreally beautiful.